1. Field of the Invention
This invention relates to methods of forming stiffened panels from materials having superplastic characteristics, and more particularly to methods of forming aerospace structural components having an internal cellular structure by the processes of superplastic forming and diffusion bonding.
2. Discussion of Prior Art
Metals having superplastic characteristics, such as titanium and many of its alloys, have a composition and microstructure such that, when heated to within an appropriate range of temperature and when deformed within an appropriate range of strain rate, they exhibit the flow characteristics of a viscous fluid. The condition in which these characteristics are attained is known as superplasticity, and, in this condition, the metals may be deformed so that they they undergo elongations of several hundred percent without fracture or significant necking. This is due to the fine, uniform grain structures of superplastically formable metals which, when in the condition of superplasticity, allow grain boundary sliding by diffusion mechanism so that the individual metal crystals slide relative to one another.
Diffusion bonding is often combined with superplastic forming to enable the manufacture of multi-sheet components of complex structure. The diffusion bonding process concerns the metallurgical surgical joining of surfaces by applying heat and pressure which results in the co-mingling of atoms at the joint interface, the interface as a result becoming metallurgically undetectable. In order to achieve structures of a complex nature it is often a requirement that the multi-sheet metals are not bonded at all their contacting areas, and therefore bond inhibitors ( commonly known as stop-off or stopping-off materials) are applied to selected areas by, for example, a silk screen printing process.
One known application using both superplastic forming and diffusion bonding is for forming stiffened components by the following method. Two sheets of superplastically formable and diffusion bondable material which will form the internal structure of the finished component, hereafter referred to as the core sheets, are selectively interlaid with stop-off material. Two further sheets of superplastically formable and diffusion bondable material are positioned on each side of the core sheets; these sheets will form the outer surface of the finished component and are hereafter referred to as the skin sheets. The "pack" of four sheets is then positioned in a form tool and placed in a heated platen press which is heated to 930.degree. C. An inert gas is injected into the space between each skin sheet and its adjacent core sheet. The pressure exerted by this gas causes the skin sheets to bow outwards and conform to the shape of the cavity of the form tool while at the same time causing the core sheets to be diffusion bonded in the areas where stop-off material is not applied. When these steps have been completed, a gas is injected into the spaces between the core sheets where they are not diffusion bonded. The pressure exerted by the gas causes the core sheets to be moved apart and form substantially rectangular cells which occupy the space between the skin sheets. These cells are formed by the continued application of pressure from the gas which causes parts of the surfaces of the core sheets to become parallel and adjacent to the skin sheets and to be diffusion bonded to them to form cell ceilings and floors while at the same time causing other parts of the surfaces of the core sheets, which, due to forming, are now vertically adjacent to one another, to also be diffusion bonded to form cell walls.
Titanium, in sheet form, has in its received state the characteristics needed for superplastic forming, and because it will absorb its own oxide layer at high temperature in an inert atmosphere to provide an oxide-free surface, it is also particularly amenable to diffusion bonding under pressure. The optimum temperature for this self-cleaning is 930 degrees Centigrade which is also the optimum superplastic forming temperature. Thus, superplastic forming and diffusion bonding of titanium components can be carried out at the same time.
The ability to combine superplastic forming ( S P F ) and diffusion bonding ( DB ) has enabled our company to design multi-sheet components of complex structure that are essentially of one-piece construction. We have described above the known method of manufacturing titanium cellular structures comprising monolithic skinned panels with a vertical I-section rib/spar internal structure. Such structures have a potential application in aircraft manufacture, for example the manufacture of wing leading and trailing edge control surfaces and canards which must have smooth, aerodynamic skin surfaces and be strong and light in weight.
Many aircraft components require structures of variable gauge. Attachment points on wing sections, for example, often require locally thickened regions.
Previously such thickening or strengthening has been achieved by using uniformly thickened sheets in the multi-sheet SPF/DB process. However, it is then necessary to add an additional, onerous step to the production process of chemical-milling in order to reduce the thickness of the sheets in areas where strengthening is not required. This chemical milling step is time consuming, wasteful of material and produces hazardous waste products.
In our co-pending patent application number GB9103804.2 we describe a method of forming a component in an SPF/DB process with selectively thickened areas in order to obviate the chemical-milling step. The process is essentially the same as that described above, with the exception that the additional step of overlaying and attaching one or more additional sheets of material to the interior surfaces of the skin sheets in areas where extra thickness is required prior to superplastically forming the skin sheets of the pack to the desired shape. In this process the additional sheets are made of the same material as the other sheets of the pack.
As is well known in the field of metallurgy, metal matrix composites based on a particular alloy exhibit qualities of improved tensile strength, wear resistance and hardness relative to the alloy alone. An example of a method of manufacture of a metal matrix composite is given in U.S. Pat. No. 4,968,348 (Abkowitz et al. ). Despite their desirable qualities, it has hitherto not been possible to use metal matrix composites in the SPF/DB production process of selectively thickened components because the composites are not superplastically formable.